Contamination control is crucial to the yield of a semiconductor integrated circuit (IC) or flat panel display fabrication process. Detection of contamination (including foreign particles, fingerprints, scratches, pinholes, etc.) is necessary prior to the exposure of a mask, wafer or glass substrate.
Apparatuses for the detection of particulate contamination in the existing photolithography tools usually utilize a dark-field scattering technique as explained in FIG. 1, in which a radiation beam 11 emanated from a radiation source 10 is scattered on contaminants on a mask 40 carried on a mask stage 30, and scattered radiation therefrom is collected by a detector 20 as a test signal. However, such a detection apparatus is susceptible to crosstalk from mirrored particles (particularly in case of a chrome bottom surface of the mask) as well as crosstalk from a pattern on the mask bottom surface (as shown in FIGS. 2 and 3 which illustrate experimentally captured original images), which may significantly degrade the signal-to-noise ratio (SNR) of the test signal and hence the detection accuracy. For example, particles present on the chrome bottom surface of the mask in which the pattern to be exposed is formed will be mirrored by the chrome surface into virtual images called mirrored particles which can undesirably cross talk with the test signal.
In order to overcome this issue, a solution has been proposed in which a theoretical configuration as shown in FIG. 4 is determined based on an analysis on the impact of crosstalk from the pattern and mirrored particles on an angle of incidence 50, an angle of collection 60 and an illumination field of view (FoV). However, in this solution, the configuration fails to take into account all possible angles of incidence 50 and collection 60. When the angle of collection 60 is very close to the angle of reflection, as shown in FIG. 5, the reflected radiation 13 will overlap an edge of an FoV of the imaging module 20 and intensify the scattered radiation 12 to be collected, leading to inaccurate determination. Additionally, in this solution, an optical axis of the imaging module 20 must be aligned with the angle of collection 60 which, however, is subject to constraints from other system parameters. Therefore, the supporting, assembly and clamping of the imaging module 20 must be adjusted whenever there is a change in the angle of collection 60. Thus, the compatibility of this solution is inferior.